USB OTG and AHB clock

I would like to know that is there any minimum value of AHB if USB OTG is needed?The MCU that I'm using is STM32F105/7 which is in connectivity line series.When I set my SYSCLK to 48MHz or 72MHz and AHB prescaler is 1, my computer can detect my device.However, when I set the prescaler to 2 and SYSCLK is 48MHz, computer shows not recognize the USB device.I also tried with other setting:1. SYSCLK: 72MHz, AHB prescaler: 2 = Ok2. SYSCLK: 72MHz, AHB prescaler: 4 = Not recognize3. SYSCLK: 25MHz (directly from HSE), AHB prescaler: 1 = Not recognize